The monitoring of rotating machinery and gears using vibration measurements for determining the operating performance and condition is an established process in many industries. Vibration measurements are commonly acquired for test data processing using accelerometers placed on the rotating machinery to evaluate the condition of rotating equipment. Monitoring methods have been applied to the preflight evaluations of rocket engines. Effective monitoring tools are needed for preflight testing of rocket engine turbopumps. With the current trend toward reusable launch vehicles that will require the turbomachinery to operate for extended periods of time and on multiple missions, active monitoring of the condition of the internal components of the rocket engine becomes more important. In an effort to increase performance, expendable launch vehicle rocket engine turbopumps are sometimes operated at speeds and loads for which the engines were not initially designed. The consequence of this increased loading is that structural margins are decreased and the potential for hardware damage or catastrophic failure increases. For a particular expendable launch vehicle engine, two cases of liquid oxygen gear damage, including a catastrophic failure, have been observed during acceptance and development ground testing of the hardware. In order to mitigate the risk associated with the decreased structural margins, a drive train diagnostic procedure is needed.
In order to gain insight into the behavior of a rocket engine prior to flight, vibration response data is acquired during acceptance tests known as static firings or hot runs. During hot runs, the engine is fixed in a test stand and ignited. The steady-state data acquired is then analyzed to determine quantitative parameters that are used to assess the vibration signature of an engine. During rotating machinery analysis, particular vibration signatures are related to specific types of component defects. For example, discrete gear tooth defects are often characterized in the frequency domain by the appearance of spectral components at higher order harmonics of the speed of the shaft upon which the faulty gear is located.
The simplest fault detection techniques use the change in statistical properties of the vibration signal as a measure of engine health. Relevant vibration parameters that have been used include both the root mean square value and the kurtosis. While these vibration parameters provide a single number that can potentially indicate a defect in the system, the vibration parameters can not identify the source leading to a change in vibration level. Some gear fault detection methods use an analytic envelope signal to provide information on the modulation of the gear mesh frequency. However, due to the extremely high operating speeds of rocket engine turbopumps, measurements of the vibration responses up to the gear mesh frequency are often beyond the capability of the data acquisition instrumentation. More recently, wavelet transforms have been used for gear fault detection.
There are several problems associated with current methods of testing rocket engine turbomachinery. Current methods of testing rocket engine turbomachinery are often used to monitor bearing conditions and typically use trend analysis with a one-sided cepstrum analysis. These methods do not provide quantitative parameters for monitoring rocket engine gears developed from the one-sided cepstrum analysis. For turbomachinery with operating ranges typical of those experienced in rocket engines, the resolution of the one-sided cepstrum analysis approach yields results that are not as easily interpreted.
In conjunction with vibration measurements acquired during tests or operation of a rotating machine, digital signal processing techniques are used in condition assessment procedures. Analysts have used a one-sided autospectral density and the one-sided cepstrum for computation purposes to indicate gear performance. The one-sided cepstrum method in particular, has been used to detect damage in both rolling element bearings and gears. Partly due to the susceptibility of engine and transmission components to fatigue failures, there has been research directed at effective detection of gear tooth damage.
The typical practice in most machinery analysis is to establish a baseline for a specific machine and subsequently implement a regular monitoring schedule. Typical rotating machinery can be compared to a baseline over an expected operating life measured over many years. In a trend analysis process, changes in relevant parameters are then tracked over the life of the machine. However, the life of an expendable launch vehicle rocket engine turbopump, including acceptance testing and operational missions, is measured in minutes. The turbopump is usually test fired at least twice prior to delivery to the customer in order to show that acceptable performance limits are met. These test firings generally last for several minutes each. Due to limited available engine life, it is desirable to perform as few tests as possible to assure nominal performance. From a diagnostics perspective, a consequence of only two hot fire tests is a limited amount of operating time of the engine during which to assess small changes in the vibration signature rendering trend analysis ineffectual.
The inherent difficulty of comparing different pieces of hardware are mitigated by maintaining a database of previous comparisons between known hardware health and associated vibration characteristics. For example, the variability in vibration characteristics for engines that perform in a nominal fashion can be established with some simple statistics. Additionally, when a correlation has previously been established between documented hardware damage and a unique vibration signature, correlation can be useful in providing an early differentiation between a nominally operating engine and one that contains a defect.
In many methods, the comparison of measurements between different machines is not recommended because of variability in transmission path effects due either to manufacturing tolerances or differences in the instrumentation setup. Also, infant operational characteristics of a machine may not apply across an entire class of machines. The constraints imposed by the methods applied to rocket engine turbomachinery make a comparison between engines impracticable. Rotating machinery can be compared to a baseline operation over an expected operating life measured in years, but life time baseline comparisons are unsuitable for an expendable launch vehicle rocket engine turbopump life, including acceptance testing, that is measured in minutes. While trend analysis is performed from hot fire to hot fire on a typical engine, trend analysis is not applied to compare specific engine parameters to other engines parameters of known operating conditions. While traditional trend analysis may be performed on a single engine, parametric cepstrum analysis has not been accurately used to compare vibration signatures across a class of engines. While cepstrum analysis has been used to perform trend analysis for a particular engine, an easily interpreted parametric database containing cepstrum parameters for both healthy and faulty engines has not been used to provide accurate indication of engine health. These and other disadvantages are solved or reduced using the invention.
An object of the invention is to provide a test method for rotating machinery.
Another object of the invention is to provide a method for detecting anomalous gear performance in rocket engine turbomachinery during acceptance hot firing.
Yet another object of the invention is to provide a method for detecting anomalous gear performance in rocket engine turbomachinery during acceptance hot firing using two-sided cepstrum analysis.
Still another object of the invention is to provide a method for monitoring rocket engine turbomachinery using two-sided cepstrum analysis for generating a quantitative parameter indicating the performance of the turbomachinery.
A further object of the invention is to provide a method for monitoring rocket engines during hot firing using two-sided cepstrum analysis for generating a quantitative parameter indicating the performance of the turbomachinery relative to a class of like turbomachinery.
Still a further object of the invention is to provide a turbopump vibration diagnostic routine that indicates the nature of a defect by monitoring the change in output characteristics of accelerometers measuring gear box vibrations.
Yet another object of the invention is to provide a method that uses vibration measurements acquired during static firing tests to develop screening parameters that are related to the condition of turbomachinery components.
The invention is a method directed towards a diagnostic procedure for measuring the performance of a machine having periodic movement that can be externally detected through acquired vibration signals. The method is based on a double or two-sided cepstrum analysis that can be applied in the preferred form to steady-state gear box accelerations. The gear box may be part of a rocket engine under hot fire testing. A cepstrum is defined as the inverse discrete Fourier transform of the logarithm of the two-sided autospectral density of the vibration measurements. The vibration measurements used in the analysis are acquired during static hot fire tests from accelerometers mounted on the external surface of the turbopump gear box of the rocket engine. Following the ground tests, the cepstrum analysis is used to provide an indication of turbopumps that have functioned normally and those with anomalous vibration signatures. The effectiveness of the method is demonstrated by comparing analysis results from an engine in good condition with a similar engine that suffered complete gear failure during development testing. The cepstrum method detects anomalous vibration characteristics and provides an indication of the type of defect. Measurement processing provides unique spectral characteristics that indicate the presence of a gear fault.
Measurements acquired during ground testing of a rocket engine are preferably converted into a parameter that is indicative of anomalous behavior of a turbopump gear under test. During ground test operations, measurements from the accelerometers mounted on the exterior of the gear box and a tachometer transducer within a pump housing are acquired on a frequency modulated analog recording tape. Following a series of tests, discrete time periods of data segments are stored in a computer in digital format. The stored measurement data is converted into a single quantitative parameter that may be indicative of anomalous behavior of a gear. The system operates on the series of stored data records, using a digital signal processing system to produce parameters that are stored in a database. These parametric results are used to assess the possibility of the presence and progression of a gear tooth fault in the gear train of the turbopump of the rocket engine.
The stored data records consist of measurements from the gear box accelerometer and outputs from a tachometer transducer. The accelerometer measures the gear box surface acceleration while the tachometer provides a signal that is periodic and is related to the rotation rate of the gear. These discrete data records are acquired at successive times during the ground test operation of the engine and each record is individually analyzed by the digital signal processing system. The digital signal processing system implements a two-sided cepstrum analysis, rotational speed detection and periodic cepstrum peak detection. A two-sided cepstrum analysis calculator converts the time records into a two-sided frequency spectrum using a discrete Fourier transform, an autospectral density calculator and a logarithmic converter, and then converts the two-sided frequency spectrum into a cepstrum vibration signal within a two-sided periodic time domain spectrum. The rotational speed detector converts the periodic tachometer signal into an average gear speed for the time keeping of the data records being analyzed. The peak detector automatically locates, within a specified periodic time range, the peak value of the vibration signal synchronized with the inverse of the calculated average gear speed. This peak value is a cepstrum parameter that is used to assess gear health and is stored in a database. A progressive increase in the value of this cepstrum parameter for a particular engine is indicative of progression of a gear tooth defect. This parameter can be compared with similar values of other like engines that have been previously associated with known gear hardware health. The two-sided cepstrum analysis method detects anomalous vibration signatures and provides a diagnostic indication as to the nature of a defect so that proper corrective actions can be rapidly implemented. Additionally, the cepstrum method provides a quantitative parameter related to a known gear anomaly. This cepstrum parameter is preferably stored in a database and used for comparison to the same parameter obtained from tests on other engines in the same class. The cepstrum analysis is preferably implemented as the inverse discrete Fourier transform of the logarithm of the two-sided autospectral density of the vibration measurements.
A single quantitative cepstrum parameter is generated for indicating gear condition. The cepstrum parameter is automatically calculated from the stored time records. The two-sided cepstrum analysis has the mathematical advantage of increased resolution in the periodic time domain particularly suitable to rocket engine turbomachinery operating at high speeds. The maintained database and quantitative nature of the cepstrum parameter allow for preflight acceptance or rejection of the turbomachinery. The method is particularly effective using data acquired from the exterior of a turbopump gear box during actual ground test operations of rocket engines as a gear condition monitoring method. The measurements acquired on the exterior of the gear box are used to assess the health of the internal rotating gears that can not be inspected after the ground tests and prior to flight. This is advantageous in assessing the risk of catastrophic engine failure during the ascent launch phase of a launch vehicle. If a high risk condition exists, the engine can be rebuilt with a new gear train potentially saving human lives and expensive hardware. The method can be applied on any turbopump with a gear train, and can be adapted to monitor other types of turbopump components, such as bearings, and could also be made to operate in real-time enabling in-flight monitoring of the gear train. Quasi-real-time calculations can be used for monitoring during ground tests for allowing an operator to shut down the test before a gear-induced catastrophic failure.
The method offers an ability to detect and quantify periodic structures in the frequency domain. The ability of the cepstrum parameter to diagnose discrete gear tooth defects is done by comparing the cepstrum parameters for several nominally operating turbopumps to the corresponding values for engines that exhibit catastrophic failure and excessive gear wear and tooth chipping. The mean value and mean plus or minus one sigma values respectively for the cepstrum value for all nominally operating engine liquid oxygen cepstrum parameters are also preferably stored in the database. There may be variation in the value of the cepstrum parameter when different engines are compared to one another. This variation is not unexpected and is a result of engine-to-engine hardware variability. Possible contributors to this variation are residual imbalances in the liquid oxygen shaft or slight gear mesh imperfections. In a preferred form, the method may be used to screen engines against in-flight gear failure during a mission by evaluating the preceding ground test vibration signatures. The primary requirement is to provide an accurate technical evaluation for the unique vibration signature associated with the progression of a defect toward failure. A secondary aspect is to describe this unique vibration signature using the quantitative cepstrum parameter that can be tracked over the operating life of a turbopump that is also stored in a database and compared to other engines. The two-sided cepstrum method is an effective gear box monitoring tool for the preflight testing of a turbopump for engine-to-engine comparisons using the database for comparing hardware health for specific vibration characteristics. These and other advantages will become more apparent from the following detailed description of the preferred embodiment.